(1) Field of the Invention
The present invention relates to a communication control which provides an ABR (available bit rate) service function to an ATM-based network, as well as an ATM (asynchronous transfer mode) switching system including the communication control device. In particular, the communication control device has a VS/VD (virtual source/virtual destination) function, and the ATM switching system includes the communication control device.
(2) Description of the Related Art
The framework and specifications of the ABR service have been provided by the ITU-T (International Telecommunication Union-Telecommunication Standardization Sector) and the ATM Forum. The ABR service is intended to provide the status of use of network resources by the transmission of RM (resource management) cells over ATM-based LAN or WAN networks, in order to minimize the intensity, spread and duration of congestion once congestion has already occurred. Hereinafter, the resource management cells used for the ABR service will be called the RM cells.
FIG. 4 shows a conventional ATM switching system.
As shown in FIG. 4, in the conventional ATM switching system, a communication control device (CCD) 1, a communication control device (CCD) 2 and an ATM switch 3 are provided, and these elements of the system are connected by forward transmission lines 30, 31, 32 and 33 and backward transmission lines 34, 35, and 37.
The CCD 1 includes a CPU (central processing unit) 11, an SETLS block 12, an ATMCP 13, a QBIC block 14 and an MACAO block 15. The CCD 2 includes a CPU 21, an SETLS block 25, an ATMCP 24, a QBIC block 23 and an MACAO block 22. The SETLS blocks 12 and 25 provide a physical layer interface function. The ATMCP blocks 13 and 24 provide UPC (usage parameter control), NPC (network parameter control) and OAM (operations, administration and maintenance) functions. The QBIC blocks 14 and 23 provide quality-of-service control and VD/VS functions. The MACAO blocks 15 and 22 provide the interface with the ATM switch 3. In each of the CCD 1 and the CCD 2, the SETLS block, the ATMCP block, the QBIC block and MACRO block are formed on an LSI (large-scale integration) chip.
In the conventional ATM switching system of FIG. 4, user data cells sent by a source terminal (not shown) are delivered to the CCD 1 via the transmission line 30. Further, the user data cells are passed through the CCD 1, the ATM switch 3 and the CCD 2 via the transmission lines 31 and 32 in a forward direction only, and they appear on the transmission line 33. Then, the user data cells are delivered to a destination terminal (not shown) via the transmission line 33.
Similar to the user data cells, RM cells are transmitted by the source terminal toward the destination terminal once every time a given number of user data cells have been sent. The RM cells sent by the source terminal are passed through the CCD 1, the ATM switch 3 and the CCD 2 via the transmission lines 31 and 32 in the forward direction. When each of the RM cells is delivered to the destination terminal via the transmission line 33, the RM cell is looped back to the source terminal by the destination terminal via the transmission lines 34-37 in a backward direction.
In the conventional ATM switching system of FIG. 4, the CCD 1, the ATM switch 3 and the CCD 2 are provided between the source terminal and the destination terminal. When one of the CCD 1 and the CCD 2 is placed into a congested condition, the congested device overwrites network resource information, such as a congestion indication, to each of the RM cells passed through the congested device, so that the congestion indication is notified to the source terminal by the looping back of such RM cells over the ATM network.
In an existing ABR service and an existing VS/VD function, the flow of the RM cells over the network is controlled on a path-by-path basis, including paths within the ATM switch.
For example, suppose that a transmitting buffer of the destination-end CCD 2 of FIG. 4 is at present in a congested condition. In such a case, it is desirable to minimize the intensity, spread and duration of the congestion by lowering the rate of the transmission of the cells delivered to the destination-end CCD 2. To achieve this objective, it is necessary to notify the source-end CCD 1 or the source terminal, as early as possible, that the congestion has occurred in the destination-end CCD 2.
However, the existing ABR service as provided by the conventional ATM switching system of FIG. 4 requires much time and many network resources to notify the CCD 1 or the source terminal of the occurrence of congestion at the CCD 2. After the congestion indication is written to the RM cells, the routing of such RM cells over the entire ATM network is necessary, and the notification of the congestion indication to the source terminal must be subsequent to the end of the looping back of such RM cells.
In addition, in the existing ABR service as provided by the conventional ATM switching system of FIG. 4, the flow of the RM cells over the network is controlled on a path-by-path basis. Even if the congested communication control device transmits a congestion indication directly to the source terminal by broadcasting, a speedy transmission of a large amount of information is necessary. For example, suppose that a rate of the cell transmission for every path connection is 100 cells per second, and 1000 path connections are established for a transmission line connected to the communication control device. In such a case, 100 cells per second xc3x971000 yields about 42 Mbps. The bandwidth available on the transmission line is narrowed, and the processing capability of the communication control device is significantly lowered.
An object of the present invention is to provide an improved communication control device in which the above-mentioned problems are eliminated.
Another object of the present invention is to provide a communication control device which provides an ABR service function to an ATM-based network, and speedily and efficiently transmits a congestion notification to a source-end communication control device or a source terminal, so as to minimize the intensity, spread and duration of congestion once congestion has already occurred.
Still another object of the present invention is to provide an ATM switching system including a communication control device which provides an ABR service function to an ATM-based network, and speedily and efficiently transmits a congestion notification to a source-end communication control device or a source terminal, so as to minimize the intensity, spread and duration of congestion once congestion has already occurred.
The above-mentioned objects of the present invention are achieved by a communication control device including: an RM cell receiving unit which receives an incoming RM cell from a source terminal via a forward transmission line; and a point-to-multipoint RM cell transmitting unit which overwrites, when a congestion at a downstream portion of an ATM-based network is detected, a congested-condition ER value and a congestion notification data to the incoming RM cell received by the RM cell receiving unit, and transmits a point-to-multipoint RM cell, having the congested-condition ER value and the congestion notification data, to an upstream communication device via a backward transmission line, so that a state of the congestion is notified to the upstream communication device.
The above-mentioned objects of the present invention are achieved by a communication control device including: a point-to-multipoint RM cell receiving unit which receives a point-to-multipoint RM cell from a backward transmission line, the point-to-multipoint RM cell having an ER value and a congestion notification data; an ER computing unit which performs an allowed cell rate ACR calculation based on the ER value output by the point-to-multipoint RM cell receiving unit, in accordance with a predetermined equation; and a shaping unit which transmits an outgoing RM cell to a destination terminal via a forward transmission line, the cell transmission of the shaping unit being controlled based on a result of the ACR calculation output by the ER computing unit, and, when a congestion at a downstream portion of an ATM-based network is detected based on the received RM cell, the shaping unit acting to lower a rate of the cell transmission according to the result of the ACR calculation.
In the communication control device of a preferred embodiment of the present invention, the point-to-multipoint RM cell transmitting unit overwrites, when a congestion at a downstream portion of an ATM-based network is detected, a congested-condition ER value and a congestion notification data to the incoming RM cell received by the RM cell receiving unit, and transmits a point-to-multipoint RM cell, having the congested-condition ER value and the congestion notification data, to an upstream communication device via a backward transmission line, so that a state of the congestion is notified to the upstream communication device. Hence, the communication control device of the preferred embodiment is effective in speedily and efficiently transmitting a congestion notification to the upstream communication device, thus minimizing the intensity, spread and duration of congestion once congestion has already occurred.
Further, in the communication control device of a preferred embodiment of the present invention, when a congestion at a downstream portion of an ATM-based network is detected based on the received RM cell at the point-to-multipoint RM cell receiving unit, the shaping unit acting to lower a rate of the cell transmission according to the result of the ACR calculation. Hence, the communication control device of the preferred embodiment is effective in minimizing the intensity, spread and duration of congestion once congestion has already occurred.